This patch fixes the following error with Clang-6.0

```
jerryscript/jerry-core/api/jerry.c:1527:71: error: implicit conversion loses integer precision:
      'jerry_regexp_flags_t' to 'uint16_t' (aka 'unsigned short') [-Werror,-Wconversion]
  jerry_value_t ret_val = ecma_op_create_regexp_object (ecma_pattern, flags);
```

Also change the `jerry_create_regexp` and `jerry_create_regexp_sz` functions' `flags` parameter to `uint16_t` since the values created with `bitwise inclusive OR` are not part of the enum.

JerryScript-DCO-1.0-Signed-off-by: Daniel Balla dballa@inf.u-szeged.hu